The invention relates to laryngeal masks, illustratively of the varieties disclosed in U.K. Patent 2,111,394B (corresponding to U.S. Pat. No. 4,509,514) and in published U.K. Patent Application No. 2,229,367A (corresponding to U.S. Pat. No. 4,995,388). Such masks are artificial airway devices designed to facilitate lung ventilation in an unconscious patient by forming a low-pressure seal around the laryngeal inlet. A seal surrounds an appropriately shaped mask which fits into the lower pharynx and is attached to a tube which emerges from the mouth, as for connection to medical gas-supply tubing.
In practice, these devices have been successful and are now in daily use in hospitals throughout the United Kingdom. Such masks have been found effective in achieving a reliable airway, preventing obstruction in the unconscious patient. As presently used, such masks are especially effective in cases where difficulty with the airway is experienced. For example, the mask has been found to prevent contamination of the lungs by blood or debris during surgery of the nose or throat. But it has become apparent that an important contraindication to its use is the patient who is at risk from vomiting or regurgitation of stomach contents while unconscious. Although the device forms an inflatable-cuff seal around the laryngeal inlet sufficient to permit inflation of the lungs during artificial ventilation, the seal is not sufficient to prevent lung contamination in the event of retching, vomiting or regurgitation. Patients who are not adequately starved prior to surgery are thus not suitable for use of the laryngeal mask. In such patients, an endotracheal tube is still regarded as affording the safest protection to the patient's airways. However, insertion of an endotracheal tube is not always without difficulty, and failure to make a timely insertion can lead to death or brain damage. In such cases, the laryngeal mask has proven to be life-saving. And, in cases when it has not been possible to safely insert an endotracheal tube, it has been found possible first to install a laryngeal mask, and then to use the tube of the laryngeal mask as a guide, for piloted insertion of an endotracheal tube through the mask.
My U.S. Pat. No. 4,995,388 describes other means associated with a laryngeal mask to prevent aspiration of stomach contents into the lungs. In essence, such means rely upon a combination of improved peripheral continuity of seal pressure against the larynx and the provision of drainage tubing for conduct of gastric contents away from the laryngeal inlet.
Regardless of the specific purpose to be achieved with laryngeal masks in use today, the problem of sealing effectiveness persists, in respect of assuring against entry of a gastric discharge in the laryngeal inlet. My U.S. Pat. No. 4,995,388 describes laryngeal-mask configurations with three different embodiments to facilitate drainage of gastric contents behind the mask. In one of these embodiments a drainage tube passes directly into the oesophagus; in a second of these embodiments, a drainage tube is forked anteriorly of the mask, with drainage openings on both sides of the mask bowl; and in the third embodiment, the drainage tube passes behind the mask to terminate alongside the mask tip.
But each of these three embodiments has been found to be less than satisfactory, for one or more reasons. The disadvantage of the first embodiment is that it is more invasive and breaks the seal which a non-draining inflatable mask establishes at the upper oesophageal sphincter. The disadvantage of the second embodiment is that the drainage provision is so positioned that it will draw off regurgitated fluid only after it has already entered the bowl of the mask, thus risking entry into the trachea of at least some of this fluid. And the disadvantage of the third embodiment is that it must be truncated diagonally at its tip to facilitate mask insertion, thus presenting an elliptical opening behind the mask tip, so that mask inflation tends to press this opening against the wall of the pharynx, thereby blocking the opening and impairing the desired drainage function.
Aside from the difficulties noted above in respect of proposals for preventing gastric contents from entry into the laryngeal inlet, it is important to note some of the environmental problems encountered by a laryngeal mask, however well-designed for accuracy of fit to the lower pharynx, so that when inflated, pressure is exerted on all surrounding structures, to greater or lesser degrees, depending on the resistance they offer to displacement. In broad terms, the structures in front of the mask are cartilaginous; those surrounding the mask are muscular; and those behind the mask are bony. Inflation of the cuff of the mask therefore results in the cartilaginous structures of the larynx being pushed forward, away from the bony structures of neck vertebrae, thus stretching and tensing the surrounding muscles. The seal generated against the laryngeal inlet therefore depends to some extent upon stretch resistance of muscles and soft tissues, as cuff-inflating volume of the mask expands.
If an unconscious patient retches, vomits or regurgitates gastric contents, the balance of forces reacting on the inflated cuff is transiently upset. The muscles surrounding the mask relax, and the sphincteric mechanism at the lower end of the mask also relaxes, with consequent loss or degrading of seal effectiveness; and fluid or semisolids can be forced upward through the oesophagus during such relaxation. Moreover, the larynx is itself displaced upwards as part of the vomiting or retching reflex, and such displacement alone may be sufficient to disrupt the seal of the mask around the laryngeal inlet.
Still further, and of crucial importance, is the fact that the airway tube to which the mask is attached offers resistance to flexure when in its normal position in the patient's pharynx. The airway tube thus exerts a constant force against the bony posterior walls of the pharynx. This means that in the event of a relaxation of the surrounding structures, the mask will tend to remain firmly in contact with the posterior wall. Vomited or regurgitated matter may therefore pass in front of the mask and so enter the laryngeal inlet.